Traditionally, endoscopic imaging devices have used a bundle of glass light fibres to deliver light from a high-intensity light source to the distal end of the imaging device for illuminating the subject. More recently, plastic light fibers have been used in some devices because of advantages in material and processing costs, reliability, ruggedness and, in some cases, performance.
Plastic light fibres are easily damaged or destroyed if they are directly connected to most light sources for glass light fibres. Plastic light fibres typically begin to soften at 70.degree.-80.degree. C. The most efficient light sources used in medical endoscopy employ xenon arc lamps that are powerful emitters of non-visible energy, i.e., infra-red and ultra-violet, as shown in FIG. 1. High performance plastic light fibres are efficient transmitters in the visible region of the spectrum but are highly absorptive outside the visible region, as shown in FIG. 2. Consequently, damage to plastic light fibres directly coupled to a high-intensity light source results from the light fibres absorbing energy outside the visible region of the spectrum.
To prevent damage to plastic light fibres, known endoscopic illumination systems using plastic light fibres usually connect plastic light fibres to the high intensity light source via a light guide using glass light fibres. This arrangement reduces the likelihood of damage to the plastic light fibres because of its relatively low coupling efficiency (for example, a power loss as high as two-thirds of the incident radiation has been measured) and because the glass light fibres thermally isolate the proximal ends of the plastic light fibres from the light source. Although the interface between the distal ends of the glass light fibres and proximal ends of the plastic light fibres may still get very warm to the touch, the temperature is usually low enough to prevent damage to the plastic light fibres. The disadvantage of this arrangement is that it provides a considerably lower light output than directly coupling the plastic light fibres to the high-intensity light source.
Because of new applications requiring a small, low cost light delivery system to produce higher illumination intensities, it is highly desirable to enable plastic light fibres to be directly coupled to the high-intensity light source.
Most known direct-coupled endoscopic illumination systems contain filtering to remove infra-red energy from the output of the light source. However, the types of dichroic type filter most typically used allow significant amounts of ultra-violet and residual infra-red energy to reach the proximal ends of the light fibres. All the non-visible radiation passing through the filter is absorbed in the first few centimeters of the plastic light fibres. The resulting heat, if not conducted away, causes damage to the light fibres by the mechanisms described above.
In addition to heating by infra-red absorption by the light fibres, heat reaches the light fibres by absorption and conduction in known arrangement. The light fibres are normally attached to the light source using a metal connector. The light beam delivered to the proximal end of the light fibres by the source optics typically has a larger extent than the extent of the bundle of light fibres. Some of the stray light overspilling the light fibres falls on the material of the connector to which the light fibres are attached and on conductive material surrounding the connector. The stray light is absorbed and turned into heat that is conducted to the plastic light fibres and into the adhesive bonding the light fibres to the light fibre connector.
Finally, known arrangements use radiation-absorbent adhesives to bond the light fibres to the metal connector. The radiation (visible and invisible) absorbed by the adhesive is dissipated as heat that further contributes to the temperature rise of the light fibres.
The combination of conducted heat, heat resulting from the absorption by the light fibres of non-visible radiation, and heat resulting from absorption by the adhesive can ultimately raise the plastic light fibres to a temperature that will severely damage or destroy the plastic light fibres and the adhesive used to hold the fibres in the light fibre connector or ferrule.